1. Field of the Invention
The present invention relates to a process for detection of misfires of an internal combustion engine mounted in a motor vehicle.
2. Discussion of the Background
In the scope of antipollution regulations such as the American OBD standards (On Board Diagnostic), it is required that electronic engine control systems of motor vehicles must be able to diagnose certain engine dysfunctions which influence pollutant emissions.
For this reason it is planned, in future electronic engine control systems, to install systems capable of diagnosing proper operation of the oxygen sensor or of the EGR loop (exhaust gas recirculation), wherein recognition of a malfunction which affects pollutant emissions must trip activation of less intensive modes of operation and/or must turn on a light on the dashboard to warn the driver (relaxed regulations) or must stop the vehicle (strict regulations).
In particular, the American and European standards provide for requiring detection of misfires and for identification of the cylinder or cylinders responsible. Such detection must, for example, furnish the percentage of misfires occurring in a given number of engine cycles.
The equipment designers and automobile manufacturers have therefore developed a certain number of techniques for detection of misfires.
Thus, there can be cited the accelerometer method, which comprises detecting a misfire by analysis of the variation of longitudinal acceleration of the vehicle, or the method using an oxygen-proportional sensor disposed in the exhaust line. There can also be cited the method using pressure sensors communicating with the combustion chambers, or that based on measurement of arc voltage or ionization current of the spark plugs (for controlled ignition engines).
The most commonly used method, however, is that which deduces the existence of misfires by measurement of the instantaneous speed of the crankshaft. The use thereof is actually extremely simple, since it involves merely applying software processing of the signal furnished by the angular position sensor of the crankshaft, a signal which is already used by the engine control system to control fuel injection, and so no specific equipment-related device is required for implementation of this method.
The analysis solely of the signal furnished by the crankshaft position sensor to detect possible misfires nevertheless suffers from certain disadvantages.
The method of detection of misfires is in fact based on the postulate that a misfire is manifested by a drop of the gas torque, which in turn generates a corresponding change of the instantaneous speed of the crankshaft. To identify misfires, therefore, it is sufficient to record the changes in instantaneous speed of the crankshaft.
However, the instantaneous speed of revolution of the crankshaft and of the engine flywheel integral therewith reflects not only the operation of the engine and the alternating thrust of the connecting rods under the effect of combustion of the carburetted mixture, but also the operation of the entire kinematic chain connecting the engine to the tire/ground interface.
In fact, the mechanical energy at the end of the crankshaft is transmitted to the wheels by a transmission system which traditionally comprises a clutch, a speed-change box and a differential, this transmission system possessing its own damping and stiffness. As a result, therefore, any abrupt variation of torque involving any of the elements of the kinematic chain, such as, for example, involving the vehicle""s wheels because of poor condition of the roadway surface, is fed back to the crankshaft in the form of oscillations, the magnitude of which will depend on the characteristics of the transmission system and of the perturbation.
As a result, the abrupt changes in instantaneous speed of the crankshaft are therefore caused not solely by misfires but also by all the perturbations capable of affecting the kinematic transmission chain, and therefore especially a roadway in poor condition.
For correct and exclusive identification of misfires, therefore, it seems important to be able to distinguish, among the changes of crankshaft speed, those due effectively to drops of gas torque from those having other causes, so that the latter are not counted.
To achieve this, additional strategies for deactivation of the program for detection of misfires therefore have been developed, which strategies are based on recognition of perturbations affecting the kinematic transmission chain. Thus, there can be cited strategies which use the information of wheel speed furnished by a specific sensor or else those requiring an accelerometer. There can also be cited the method described in British Patent GB A 2290870, which attempts to identify such perturbations by fuel motion in the fuel tank.
It nevertheless appears at present that none of the proposed methods is capable of distinguishing, in simple, economic fashion and with sufficient precision and reliability, the changes of crankshaft speed caused by perturbations which affect the kinematic transmission chain.
The object of the present invention is therefore to provide a process for detection of misfires which remedies the above-mentioned disadvantages.
The process according to the invention for detecting a misfire relates more particularly to multi-cylinder internal combustion engines for motor or road vehicles. This process is performed by analysis of the values of a variable representative of the gas torque of firing quality deduced from the crankshaft rotation, to detect the occurrence of a misfire.
According to the invention, the process for detection of a misfire is characterized in that detection of misfires is suspended for a predetermined period, when the analysis of these values characteristic of firing quality reveals anomalies originating from the kinematic transmission chain connecting the engine shaft with the vehicle""s wheels, this detection of misfires being suspended when the considered values characteristic of firing become higher than a given threshold known as the upper threshold.
The innovation of the invention is to deduce directly from the signal used to identify misfres the occurrence of perturbations in the transmission likely to falsify the identification of misfires.
The principle of the strategy according to the invention is based on the fact that, in contrast to misfires, which can merely cause a drop of the observed signal, the perturbations originating from the kinematic chain for transmission of engine motion to the wheels cause both drops and rises of the level of the observed signal, and do so in generally alternating fashion.
According to another characteristic of the process of the invention for detecting a misfire, said process comprises the following operations executed after each firing:
(a)xe2x80x94a value characteristic of firing is produced;
(b)xe2x80x94this value characteristic of firing is compared with the upper threshold;
(c)xe2x80x94when the value characteristic of firing is higher than the upper threshold, the detection of possible misfires is suspended for a predetermined period;
(d)xe2x80x94otherwise, this value characteristic of firing is compared with a threshold value known as the lower threshold;
(e)xe2x80x94the existence of a misfire is detected when the considered value characteristic of firing is lower than the lower threshold.
According to another characteristic of the process of the invention for detecting a misfire, the upper threshold is a function of the considered value characteristic of firing.
According to another characteristic of the process of the invention for detecting a misfire, the upper threshold is a function of the lower threshold.
According to another characteristic of the process of the invention for detecting a misfire, the upper threshold is a function of the considered value characteristic of firing and of the lower threshold, after appropriate statistical processing of these parameters.
According to another characteristic of the process of the invention for detecting a misfire, the upper threshold is predefined as a function of the operating conditions of the engine.
According to another characteristic of the process of the invention for detecting a misfire, the predetermined period during which possible detections of misfires are no longer taken into account is constant.
According to another characteristic of the process of the invention for detecting a misfire, the predetermined period during which possible detections of misfires are no longer taken into account is predefined as a function of the operating conditions of the engine.
According to another characteristic of the process of the invention for detecting a misfire, the predetermined period during which possible detections of misfires are no longer taken into account retroactively encompasses a given number of preceding firings.